Fat transplantation can have metabolic benefits

May 6, 2008

When transplanted deep into the abdomen, fat taken from just under the skin comes with metabolic benefits, or at least it does in mice, reveals a new study in the May issue of Cell Metabolism.

“We started out thinking we would find that any fat inside the abdomen is bad,” said C. Ronald Kahn of Harvard Medical School. “What we found was really not what we expected. It appears that visceral fat is not as bad as subcutaneous fat is good.”

Earlier studies had established a link between the presence of intra-abdominal, or visceral, fat and “metabolic syndrome” in humans. Increased amounts of subcutaneous fat, on the other hand, had been associated with improved insulin sensitivity and a lower risk of developing type 2 diabetes, among other benefits.

Visceral and subcutaneous fat show major differences in the genes they express, studies also showed. Given the differences in their locations in the body, the two types of fat cells are also exposed to different environments. For instance, they differ in their exposure to various hormones and growth factors and in their nutrient and oxygen supply.

“This raises the question: Are the metabolic effects associated with visceral fat versus subcutaneous fat due to anatomic location or to cell-autonomous differences between these adipose depots"” the researchers said.

To find out, the researchers used a fat transplantation strategy in which they transplanted either visceral or subcutaneous fat from donor mice into either visceral or peripheral subcutaneous regions of recipient mice and examined the effects on both whole-body and cellular metabolism.

Mice that underwent transplantation of subcutaneous fat into the visceral cavity exhibited decreases in body weight, total fat mass accumulation, and fat cell size after several weeks even though they continued to eat just as much and weren’t any more active. These mice also showed improved blood sugar and insulin levels in comparison to mice that had undergone a “sham” operation.

The positive metabolic effects were specific to transplantation of subcutaneous fat and were greatest when subcutaneous fat was transplanted into the visceral cavity, the researchers reported.

“Somewhat surprisingly, we found that the major effects on metabolism and body weight were beneficial effects of added subcutaneous fat rather than a detrimental effect of added visceral fat and that these effects were greatest when subcutaneous fat was placed in an intra-abdominal site,” Kahn said. “This suggests that fat cells in different depots have intrinsically different properties and that these may be detrimental as well as beneficial.”

The findings offer a new perspective on fat, a tissue that is so often vilified across the board. “Our findings say that there is some good fat,” Kahn said.

The researchers suspect that subcutaneous fat may produce some substance that confers a metabolic advantage, particularly when in close proximity to the abdominal organs. “Now we need to find out what it is about subcutaneous fat,” Kahn said.

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